(1) Field of the Invention
The present invention relates to AA alkaline batteries.
(2) Disclosure of Related Art
In alkaline batteries, there is the possibility of generation of hydrogen gas for the structural reasons. The generation of hydrogen gas increases the internal pressure, thus causing a hazard. In view of this, alkaline batteries are designed to prevent generation of hydrogen gas or to ensure the safety of the batteries even with generation of hydrogen gas.
Specifically, an alkaline battery uses zinc as a negative electrode active material and also uses a strong alkaline electrolyte as an electrolyte which is in contact with a negative electrode. Accordingly, the surface of zinc might be corroded by the strong alkaline electrolyte so that hydrogen gas is generated. Since the alkaline battery is hermetically sealed, generation of hydrogen gas in the alkaline battery increases the pressure inside the alkaline battery to cause a hazard to the alkaline battery. To prevent this, bismuth or indium, for example, is added to the negative electrode in the alkaline battery in order to suppress corrosion of zinc by the alkaline electrolyte. In case of an increase in internal pressure of the alkaline battery, a safety valve opens to reduce the pressure inside the alkaline battery.
Such an alkaline battery uses a brass nail-shaped member as a negative electrode current collector in many cases. Brass previously contained iron in a nonnegligible amount, specifically in an amount exceeding 100 ppm with respect to the weight of the negative electrode current collector (i.e., brass). If such a nonnegligible amount of iron is contained in the negative electrode current collector, generation of hydrogen gas might be accelerated. To prevent this, in Japanese Laid-Open Patent Publication No. 05-109411 (hereinafter, referred to as Patent Document 1), tin or indium is provided on the surface of a negative electrode current collector to mask an impurity such as iron. However, brass has high purity in recent years, thus eliminating the necessity of providing tin or indium on the surface of the negative electrode current collector in order to cover iron serving as an impurity.
In recent years, increase in capacity and power and cost reduction have been required of AA alkaline batteries. The increase in capacity of an AA alkaline battery is achieved by increasing the loading weight of an active material in the AA alkaline battery. To increase power, the content of the active material in the negative electrode is increased, i.e., the weight of materials (e.g., bismuth or indium) except for the negative electrode active material (i.e., zinc) is reduced. This is because the reaction efficiency of zinc serving as the negative electrode active material is enhanced. In addition, since indium is relatively expensive, reduction in the amount of indium leads to cost reduction of the AA alkaline battery.